In late 1990, the Institute of Electrical and Electronics Engineers (IEEE), the standards body which maintains Local Area Network (LAN) standards, revised the 802.3 Ethernet specification to add the so-called 10BASE-T specification, which allows use of unshielded twisted pair as one the media for interconnection of Ethernet nodes. Because 10BASE-T lowers the overall cost of building a local area network and provides for certain network management functions not possible with traditional thick and thin coax media, it is expected to become the predominant medium in the future, with the percentage of new nodes using 10BASE-T expected to rise from about 30% in 1991 to more than 80% by 1995.
Most current designs for an Ethernet network interface card (NIC) consist of three chips: a transceiver, a Manchester encoder/decoder and the network controller. The transceiver and the encoder/decoder are connected through the Attachment Unit Interface (AUI) port, which provides a medium independent interface. When twisted-pair (TP) was first introduced as a medium for Ethernet, a technique of predistorting the wide pulse and putting it through a 7th order elliptic filter with 3dB band edge at 15-20 MHz was used to fit it into the existing jitter budget of the network. Therefore, a TP medium attachment unit is usually implemented together with a transceiver, a hybrid transmit/receive filter and a transformer.
Testing and reliability issues of the discrete hybrid filter have made the integration of the external filter an important issue. Also, to improve overall reliability and costs, integration of the transceiver, encoder/decoder, external filter and, eventually, the network controller has become a goal for the local area network equipment designer.
One embodiment of such a precoded waveshaping technique, disclosed by LEVEL ONE COMMUNICATIONS and Fujitsu Corp. in two papers entitled 10Mb/s 10BASE-T/AUI TRANSCEIVER WITH INTEGRATED WAVESHAPING FILTER AND DATA RECOVERY, and A HIGH INTEGRATED PRODUCT FAMILY FOR 10BASE-T APPLICATIONS, respectively includes a Manchester encoded signal at its output and does not require a typical 10BASE-T 5-7th order 15 MHz elliptic passive filter. The only external components are the insertion resistors and the line isolation transformer. The waveshaping technique is based on shaping the output waveform at N times the data rate to make it appear as though it has been passed through an elliptic filter. The high frequency signal aliasing is then eliminated by a first order lowpass continuous filter. To reconstruct the significant trailing tail effect from the previous pulse, two bits of information are used to decide the proper waveshaping. Although this prior waveshaping apparatus is an improvement over previously known systems it has some disadvantages. This technique requires an active filter which consumes additional power and also provides additional complexity to the transmitter. An additional problem is that the conventional prior art waveshaping techniques use 1 to .sqroot.2 transform ratio to avoid headroom problems with output of the line driver. This ratio can cause nonlinear behavior at the output due to additional harmonics on the output signal. A final disadvantage of conventional waveshaping techniques is that a certain amount of open loop gain is required at the sample frequency. Hence, the line driver circuit will be sensitive to temperature and process variations.
Accordingly, what is needed is a precoded waveshaping transmitter that overcomes the above mentioned problems associated with conventional transmitters. The present invention addresses such a need.